Frequency modulated picture receiver



F b. 10, 1948. CARNAHAN V 2,435,736

FREQUENCY MODULATED PICTURE RECEIVER Filed Feb. 13, 1941 3 Sheets-Sheet1 IO 1| I I9 20 2: 22 /23 24' PICKUP VIDEO 7 MIXING FREQ-MOD. FREQ.EXCITER PWR- DEVICE AMPL. AMPL. OSC. MULTIPLIER M VERT. 8. HOR- VERT.SYNC. HOR. SYNC. BLANKING IMPULSE IMPULSE IMPULSE GEN. GEN. GEN.

\ F SYNCHRONIZING 7 Go GENERATOR V IllHHI-IIIIHIIHIIIII II II FIG.5

'DISCRIM.

OUTPUT INVENTOR MOD. osc. FREQ.

F G .7 C. 14 5525) CARA/AVIAN ATTQRNEY Feb. 10, 1948.

I c', w. CARNAHAN 2,435,736

FREQUENCY MODULATED PICTURE RECEIVER Filed Feb. 15, 1941 3 Sheets-Sheet2 I 38 39 /4| 34 4.3 osc,& F.M. VIDEO MIxER I DISCR. AMP. I I 49 5| I HR- HOR. V SYNC! DET. SWEEP FIG. 8 I

VERT. VERT. SYNC. DET. SWEEP PICKUP VIDEO MIXING FREQ.MOD. FREQ. ExcPWR.

DEVICE AMPL. AMPt. 05c. MULT. *AMP.

, M l i j VERT. & HOR. VERT. SYNC. HOR. SYNC. AMP.

BLANKING IMPULSE IMPULSE MOD, IMPULSE GEN. GEN. GEN. I

I W I V F H S'YNCHRONIZING l G GENERATOR I 76 77 I /7a /79 I/sl 80 05c.8. EM. VIDEO MIXER DISCR. AMF? Ti) as I as I .3 VERT. VERT. SYNC.DET.swEEP a I HOR. HOR. I SYNC. DET. SWEEP INVENTOR I I CWESLEYCAR/VAHANATTORNEY Feb. 10, 1948. c. w. CARNAHAN 2,435,736

FREQUENCY MODULATED ncwunnmcmvgn Filed Feb. 13, 1941 3 Sheets-Sheet 3 Ius SYN. DET. ami a SWEEP F I I n4 VER'E SWEEP F|cfl3 INVENTORCWE'SLEYCAR/VAHAN IBYWZQW ATTORNEY Patented Feb. 10, 1948 UNlTED STATESPATENT OFFICE FREQUENCY MODULATED PICTURE RECEIV Chalon Wesley Carnahan,Oak Park, Ill.. assignmto Zenith Radio Corporation, Chicago, 111., acorporation of Illinois ApplicationFebruary 13, 1941, Serial No. 378,715

4 Claims. (Cl. 178-73) rier wave of appreciable intensity, such asrequired for transmitting stations with power ratings in the order ofkilowatts, is accompanied with xtreme difliculties. In order to obtainreasonable efiiciency, it is conventional transmitter practice to employnon-linear amplifiers, socalled class C amplifiers, for poweramplification of the carrier waves. Since no non-linear amplification ofthe carrier wave may occur after modulation thereof, it is necessary tomodulate the carrier wave at a high power level. Such a modulationrequires considerable modulating power, and since the modulating picturesignals extend over a frequency band ranging from 60 cycles to about 4megacycles, it is extremely diflicult to obtain modulating signals ofsufficient power without encountering serious signal distortion. Forthis reason it has been proposed to efiect carrier wave modulation at alow power level. In that case the power amplifiers for increasing thepower of the modulatel carrier waves must have linear frequency responseover a frequency band of about 4 megacycles, which is difficult toobtain and inherently makes for very low efliciency of the transmitter.

It is an object of the present invention, therefore, to provide atelevision transmitting system and a receiving system for cooperationtherewith, which overcomes the above defects of conventional apparatusby employing frequency modulation, of carrier waves in accordance withthe picture signals to be transmitted.

The advantages of such frequency modulation systems will become evidentfrom the following comparison. Modulation can be effected at a low powerlevel of the signal carrier wave, therefore requiring no appreciablemodulating power. The final power amplifier of a transmitter accordingto the present invention, can be operated for highest power output, thatis, so-called class C operation, at the maximum plate dissipation of theamplifier tubes, since no attention need be paid to linearity in thefrequency modulation amplifiers. This should result inapproximately fourtimes the average power output of a conventional grid modulatedamplitude modulation amplifier stage and considerably greaterefficiency. Hence, a transmitter according to the present invention, canproduce approximately four times the average power of a conventionaltransmitter using substantially the-same complement of tubes. Areceiving antenna located at a, certain distance from the transmitteraccording to the present invention, therefore, receives four times theaverage power than from a conventional transmitter at the same distance.Hence,

the average carrier wave'voltage amplitude at the receiver isdoubledandso isthe signal to noise ratio, which considerably enhancesthe quality of th reproduced television picture.

In order to synchronize the operation of transmitter and receiver, it isnecessary to transmit synchronizing signals in addition to the picturesignals. It is conventional to transmit both types of signals by way ofthe same carrier waves. It will become evident from the following thatconsiderable advantages are also obtained if the frequency of the signalcarrier waves is also modulated by the synchronizing signals.

In order to appreciate the objects and advantages of the presentinvention, it is necessary to refer to the typical signals ordinarilyemployed in television at the present time and for this purposereference is now made to Figs. 15 and 16 which graphically depict suchtypical signals and to Fig. 10 which shows diagrammatically in fulllines and in dotted lines typical successive scanning patterns forinterlaced scanning.

In the usual television system employed at the 7 present time, a singleamplitude modulated carrier is transmitted, having an envelope of thegeneral form shown in Figs. 15 and 16. In these figures the videosignals are I 02. The blanking pulses between successive video signalsare indicated at IIII and during these blanking pulses ocour thehorizontal synchronizing pulses :03. It

is preferred to maintain the synchronizing pulses I03 at all times, thatis, during the video trans-. mission and also during the vertical returnblank ing period.

The vertical return occurs during a blanking pulse I06, equivalent inlength to several scan-- ning lines. The vertical synchronizing pulse INor IOIa occurs during the long blanking pulse,

llgence by which background control may be obtained at the receiver.

The composite signal from the mixing amplifier I9 is supplied to atransmission line 20 capable of transmitting the necessary frequencyband width. This transmission line extends from the pickup point,usually a studio, to the broadcast transmitter. The output of thetransmission line 20 is, or may be, further amplified, if desired, andapplied to a frequency modulator-oscillator 2| which provides a carrierin the usual manner to be phase or frequency modulated by the compositesignal received from the transmission line. The output of themodulator-oscillator 2| is supplied to a frequency multiplier 22, whichmagnifies the phase angle change so as to produce the'desired frequencydeviation of the carrier from its nominal or rest frequency value. Thefrequency modulated carrier produced in the frequency multiplier 22 isfed to an exciter, or driving stage 23, which drives the radio frequencypower amplifier 24. The output from this latter amplifier is transmittedto a radiating system indicated generally by the reference character 25.The output of the frequency multiplier 22 may be used, if desired, toenergize a monitor system at the transmitting station. The monitorsystem may comprise a television receiver similar to that shown inFigure 8 of the drawings.

Figures 2 to 5 of the drawings show thewave form of the signal at eachpoint of the system for an assumed horizontal scanning line, whichincreases in brightness from left to right as viewed by the pickupdevice IO when it is focused on the field of view. In Figure 2 theoutput of the pickup device is shown as an increasing potential 2B andis reproduced at higher potentials in a shorter time as the scanningmeans, which may be an electron beam, retraces the field of view fromright to left. This retrace signal 21 is undesirable and, therefore, itis removed or blotted out as indicated at 28 by an impulse from theblanking impulse generator 11, as shown by Figure 3 of the drawings.

The blanking out of the signal during the retrace period may beaccomplished by any one of a number of ways known to the art. One suchmethod is to mix the signal of Figure 2 with a large impulse occurringduring the retrace period, having an amplitude in the black directionappreciably greater than the maximum signal from the pickup deviceduring the retrace. This combined signal is then fed to an amplifyingstage the grid of which is so biased that all of the signal variationduring the retrace period lies below the cut off of the amplifier.

The signal level 29 obtained when the blanking impulse is applied may beselected to correspond to the darkest portion or portions of thepicture, which is referred to as the black level. Numerous ways ofobtaining the black value are known to the art and any of these may beemployed. The pedestal level may be adjusted at the transmitter to anypoint for economical use of the available modulating range. It ispreferably as close as possible to the maximum picture signal peaks.

Figure 4 of the drawings shows the addition of the horizontalsynchronizing impulses 30, the amplitude of which in the black directionis greater than the value of the potential for the black level 29 of thepicture, and which, therefore, do not affect the picture reproduced atthe receiver. The horizontal synchronizing impulses 30 cause thescanning means to traverse the field shown in the upper portion of thefigure.

of view from right to left in preparation for the derivation of signal26 from the next following scanning line. The series of signals shown byFigure 4 are repeated until the total area of the field has been scannedonce in alternate lines.

Figure 5 of the drawings shows the signal wave form for a completedouble scanning of the field. Following present conventional practice,this drawing represents 441 lines, each complete scanning involving 441horizontal pulses 30. Figure 5 does not disclose the signal which occursduring the main part of the picture scanning, but it will be understoodthat such full disclosure would merely embody the multiplication of theleft-hand and right-hand parts of the figure. The principal purpose ofthis figure is to show in a comparative relation the form of the signalat two successive vertical return periods.

The upper part of Figure 5 begins with the signals 26 representingscanning lines on the frame separated by blanking signals 29 andhorizontal return signals 30.

When the last line is scanned, a prolonged blanking signal 29' occursfor the'vertical return. During this blanking signal 29', the horizontalreturn pulses 3!] occur at regular rate. It will be understood that theblanking signal 29' causes the signal trace to assume the black levelindicated by the reference character 29 and to maintain this level inthe absence of additional pulses throughout a period which would becccupied by the transmission of several horizontal pulses 39. As statedbefore, the horizontal synchronization pulses 30 are continued at theirregular intervals to maintain synchronism of the horizontal oscillatorwhich might otherwise fall out of step during the period of time whenthe vertical retrace to the top of the field of view is effected.

In order to return the scanning beam to the starting position on theframe for scanning the next successive field, a vertical synchronizingpulse 32 is fed to the mixing amplifier I9 from the verticalsynchronizing pulse generator Hi. This vertical synchronizing pulse 32is preferably less than one half of a line in duration. The horizontalsynchronizing pulses 30 are of values beyond the black level 29 of thepicture. The vertical synchronizing pulse 32 is still further beyond thelevel 29, that is, it is greater in magnitude than the pulses 30-, asclearly shown in Fig. 5. The difference in magnitude between the pulses30 and 32 provides a basis for separation of these pulses at thereceiver in a novel manner.

The effect of the pulse 32 causes energization of the vertical controlplates or coils of the reproducer at the proper time with respect to thehorizontal synchronization pulses 30 which cause energization of thehorizontal control plates or coils of the reproducer, so that thehorizontal scanning of the second field will be initiated to providecorrect line relationship for interlace.

The pulse 32 is applied at regular intervals. Thus, for pictures inwhich the scanning occurs sixty times a second, the pulse 32 is suppliedeach sixtieth of a second. The two portions of Fig. 5 are related sothat the form shown below represents the condition of the signalonesixtieth of a second after the signal has the form In other words, ifany point on the upper portion is taken, that point shows theconditionof the signal at a particular time. Then the point on the lower portionof the figure immediately below 7 thatpointshows the condition of thesignal onesixt-iethof asecond after that time.

Consequently, the two pulses 32 21min vertical alignment. However, inthe two portions of the figure, owing to the odd number of lines in acomplete doublescanning, the pulses 3d are displacedrelatively halfa'line in the two portions ofthe: figure. Therefore,- each of the pulses32 in-the-twoportions of the figure and'in the corresponding verticalreturn-periods has a different relation-to the: horizontal pulses 3B.Owing, however, to my. improved method,- of transmission, thehorizontal: pulses 30. have no effect upon the result -.of thevertical-pulses 32 and consequently I am able toavoid the necessity ofthe complex systemlheretofore necessary for the introductionofequalizing. pulses.

It is to be notedthat the video signal is supplied: to the frequencymodulated oscillator at a point-of the transmitter where the power levelis stillwlow. -.Consequently,i1;.:ls not necessary to build up the videosignal-to a degree of power of thesame orderas the output signal as hasheretofore been the case withv amplitude modulatedtelevision-transmitters.

The. frequencyexcursions of the transmitter carrier resultingfrom theamplitude values of the synchronization impulses areshown in the formofa curve -36 on Figure 7,.in which the abscissae are values. of modulatedoscillator frequencies and the ordinates are the amplitudes of the'mixing-amplifier output. Point B on the curve representssthe.minimumvisual effect and correspondsto the signal level Hand 29 on Figures 3,.4.. and 5 of the drawings. -Point- W represents themaximum frequencyexcursion which corresponds-to the. maximum brightnessv obtainablein'the usual types of reproducer. This will represent maximum brillianceof the fluorescent screen or coating-in the electron ray reproducingtube 35. PointsH and-V represent the frequency excursions caused-by thehorizontal and vertical synchronizing impulses 30 and 32 respectively.

Referring to Figure 8=of thedrawings, the receiver may comprise anantenna system 31, an

oscillator and mixer stage 33, and an intermediate frequency amplifyingsystem 39, following conventional superheterodyne practice. The.amplitude variations of the composite intermediate frequency signalsare removed in the limiter stage 4|, in the manner usual in-phase orfrequency modulated receiving systems. The limiter 4| may be of anyconventional type such, for example, as the type disclosed in Frequencymodulation," by Ridenonpage 53, or the type disclosed in the copending.application of the present inventor, C. Wesley Carnahan, Serial No.371,606, filedDecember 26, 1940,.now Patent No. 2,323,880, dated July 6,1943.

, The composite signal as it comes from the limiter 4| consists of thecarrier varying in frepicture tube is provided with the usual horizontaland vertical deflecting coils or. plates. The hurlzontal and verticaldeflectingcoils or plates are energized by current orvoltage-wavesobsawtooth form, as conventional.

Referring to Figure 9,.adesirable characteristic curve of adiscriminator is shown, suitable-for use as the discriminator in thereceiverof Figure/8. A point B on this curve may be selected asthe blackvalue corresponding to the black value-29 of Figures 3 to 5 sothatamplitude responses having this value-will produce substantiallyno'visible luminescence on the fluorescent screenoof the picture tube35. The synchronizing impulses which attain amplitude-levelshaving-values-H and V indicated on the curve of Figure 7 havev no effecton the reproduced pi'cture,-as-they have values which are blacker than.black.

The intermediate frequency signal,v derived-in the oscillator mixerstage of'atyplcalspicture receiver from the wave radiated from theantenna system 25, is represented by the frequency band diagram ofFigure 6, which shows thetotal-band width through which the frequency ofthesintermediate frequency signal varies during the transmission'of thevideo and synchronizing signals. The frequency variation, or modulation,.of the intermediate frequency signal as 'thejpictureis scanned isconfined .to the frequency: range '33. It will be noted that frequencyexcursionsr beyond the frequency valuevBx will haveno efiectron thequencyinaccordance with the video signal and appearance of the picture,and, therefore; the fre'quencyspectrum beyond the point-1B is' availablefor: controlling receiver'functions. Thehorizontalsynchronizationimpulses '30 for controlling the horizontalscanning oscillator: are translated into frequency variations 'in thenarrow-band labeledtfif on Figure 6 of the drawings,'these variationsbeing displaced in frequencyfroxn the video band 33 so. as toprovideready separation therefrom on a' frequency discrimination basis in amanner to be explained in :connectionwith the operation of the receiver.The vertical synchronizing impulses 32 are translated into frequencyvariations and occupya narrow frequency band'32f which is removed infrequency value from the video signal band 33' and thenarrow band 38occupied by horizontal'impulses. This displacement in frequency of thehorizontal and vertical components of the frequency modulatedintermediate signal is obtained by reason of'the larger amplitude of thehorizontal andiverti'cal components as supplied by the respective signalgenerators. The separation and utilization of the set of synchronizationsignals is done on a frequency discrimination basis and thetotalcomposite signal demodulated by the discriminator 34 is used to excitethe picture tube 35 of the'receiver (Figure 8), the efiects of the setof synchronizing signals being eliminated by biasing the tube in theusual manner to produce no visual response when signals are impressed onthe tube which are outside of the amplitude range corresponding 't'o-ablack area of the original subject matter at the transmitter.

Intermediate frequency excursions produced by frequency modulation ofthe transmitter carrier with the horizontal and vertical synchronizingimpulses are supplied from the limiter 4| to'the horizontal and verticalimpulse separating systems 36 and ll. The separatingsystem 46 comprisesa circuit 48, whichis tuned to the interrelationship named by simplemeans.

amplitude demodulator or detector 4'9. The tuned circuit 48'acts as afilter in'the well-known manner for frequencies other than theintermediate frequency value corresponding to the frequency produced bythe horizontal synchronizing impulse. The horizontal synchronizingimpulse is obtained by demodulation in the amplitude detector 49 and isapplied to control the horizontal sweep generator 5|, which supplies thehorizontal deflecting coils or plates of the picture tube 35 with waveshaving a wave form to cause horizontal scanning at uniform velocity.

The separating system 41 comprises a circuit 52, which is tuned to thefrequency value attained when the carrier is modulated with the verticalsynchronizing impulse. The tuned circuit 52 causes the intermediatefrequency signal produced by the vertical synchronizing signal to beapplied to the amplitude detector 53. The vertical synchronizingimpulses obtained from the detector 53 are utilized to control thevertical sweep generator 54, which supplies the vertical deflectingcoils or deflecting plates of the, picture tube 35 with undulations of awave form to accomplish scanning in the manner set forth above.

A brief description of the operation of the entire system just describedwill now be given. Since the invention is especially applicable to atelevision system in which the entire frame, or field of view, isscanned in alternate lines, a system which is known as interlacedscanning, the following description of the operation of the system willdeal with interlaced scanning.

In the conventional television system employinginterlaced scanning, onenecessary relation between the several synchronizing signals used toactuate the deflecting means for the scanner is that the ratio betweenthe relatively high frequency of one synchronizing signal and therelatively' low frequency of the other synchronizing signal must be aninteger plus a fraction such as one-half, one-third, one-fourth or thelike. While the invention is not limited to interlaced scanning, itprovides means for readily and clearly separating synchronizing impulseshaving the For a clearer understanding of the interlaced scanningprocess and the co-relation between the vertical and horizontalsynchronizing signals reference may be had to Figure of the drawings.The first field is scanned beginning with point A at or adjacent to theupper left hand corner of the total field of view, the first horizontalsolid line sloping diagonally downward to the right, tracing a resultantof the horizontal and vertical scanning oscillator outputs. At B thevideo signal is blanked and the beam or other scanning means is returnedto the point D. The wave form of the vertical oscillator decreases fromits maximum value at such a rate that the second solid scanning line isdisplaced a distance of two lines on the frame. The first field ends ata point E at or adjacent to the center of the lower edge of the framefrom which point it is returned to the point F. From F, it follows thedash line path to the point G, thereby producing the interlaced pattern.If the scanning beam moves substantially horizontally, the horizontalscanning frequency will be considerably higher than the verticalscanning frequency. For good definition, simple interlaced scanning maybe employed in which two fields or interlaced sets of lines constitute aframe. There may be 441 lines to a frame and 30 frames per second. Thisrequires,

for interlaced scanning, a vertical scanning wave of 60 pulses persecond and a horizontal scanning wave of 13,230 pulses per second. Forpurposes of illustration the invention will be described with respect toa 441 line system. It will be un-, derstood, however, that the inventionis not limited to the particular values givenabove, but is applicable toother systems in which the entire area of the field oi viewlis scannedonce in substantially contacting lines.

The pick-up device in is provided in the usual manner with horizontaldeflecting means, such as deflecting plates or coils and verticaldefiection means, such as vertical deflecting plates or coils, for thescanning ray. These deflecting means are excited from thesynchronizinggenerator 18. The horizontal deflecting means are excited by anundulating voltage, or current, having what is usually known as asawtooth wave form. By reason of this special wave form, the

scanning beam will be deflected at a uniform,

speed from one side of the field 01 view to the other. As the scanningwaveattains its maximum value and decreases quickly to zero, thescanning beam will return to its starting point on the field of view.This return of the scanning beam, as pointed out above, generates asignal in the pick-up device at a somewhat higher'voltage because of theincreased speed of scanning, but this retrace signal is blotted out inthe video an;- plifier II by an impulse from the blankingimpulsegenerator ll,

As the scanning beam sweeps back and forth under control of thehorizontal impulse generator l4, it is displaced at a uniform rate byanother undulating current or voltage which is applied to the verticaldeflecting coils, orplates, which are in space quadrature with thehorizontal deflecting coils, or plates. The horizontal and verticalcontrol impulses from the impulse generators I4 and I6 are mixed withthe video signal in the amplifier I9 to form a composite wave, which isusedin the modulator 2| to vary the phase or frequency of the carrier byphase or frequency modulation. The instantaneous frequency of thecarrier is multiplied in the frequency multiplier 22 and is radiatedfrom the antenna system 25 after further amplification in the usualmanner.

The foregoing description in connection with Figure 8 of the drawingscontains a description of the operation of the receiver which need notbe repeated here. It was pointed out therein that the signal fed to thefrequency detector 35 is applied to the picture tube 35 but only theSignals derived from the frequency band 33 in Figure 6 are eilective tooperate the picture tube since frequencies lying outside of thisband-produce demodulated signalsof an amplitude which is too great, asexplained above, to effect the picture tube, since it is renderedinsensitive to signals. having amplitudes beyond the picture signalrange.

The elimination of the necessity for including equalizing impulses attwice the horizontal line frequency is an important feature of theinvention and will be explained more in detail at this point. Theequalizing impulses at twice the line frequency are added in knownpicture transmission systems in the region of the vertical synchronizingimpulses to reduce the inequality of the phase relationship, betweenhorizontal and vertical synchronizing impulses for alternate scanningfields when separation of the horizontal ii and vertical impulses isaccomplished by wave form discrimination methods. The verticalsynchronizing impulses f or odd and even fields occur at "diflerenttimes with respect to "the last horizontal pulsepreceding the verticalpulse; 'Equalizingpuis s are utilized to correct for 'diSsyinmetry ofthe horizontal synchronizing impulses with respect to the verticalsynchronizing ,impulses. When the conventional methods of synchronizingsignal separation are used, the horizontal impulses do produce atransient response of the separating circuit, thus affecting the time ofoccurrence of the point of the vertical synhro'riizing wave to whieh'thevertical synchronizer oscillator control responds. In the absence ofequalizing pulses, the efict is not uniform for odd and even fields. Toeliminate this lack of uniformity, it is necessary in prior art systemsto substitute for horizontal pulses immediately before "the verticalpulse, a series of equalizing pulses of twice the frequency of thehorizontal pulses, alternate equalizing..pulses coinciding in time withthe horizontal synchronizing pulses.

There is, therefore, an impulse which is equivalent in'value to theequalizing pulse always occur'ringat the same time interval before avertical pulse. The end result of this process in knownpicture systemsis a complex synchronization signal which, because of the variety ofinterlocking circuits required 'to produce it, adds greatly to thecomplexity of the transmitter equipment. A complete separation ofvertical and horizontal synchronizing signals on a waveform basis isimpossible without the inclusion of equalizing impulses at thetransmitter in known television systems.

The system of Figures 1 to 9, whichembodies the present inventions,eliminates the necessity for adding equalization impulses, sincecomplete separation of both signals is obtained on an amplitude andfrequency basis and it is not necessary to compensate for residualvertical signals in the horizontal signal separationland utilizationcircuits. Hence perfect synchronization can be obtainedbytransmittingtwo series of simple signals only, namely, ,a series. of'line synchronizing signals occurring at regular intervals of linescannin'gfrequency only, and a series o'ffield synchronizing signalsoccurring at regular intervals of 'field scanning frequency only.v Noris it necessary to separate the picture signals from the synchronizingsignals before the former are appl ed to the picture 'rep'roducingitube35.

In Figure 6, assuming 321 as thirteen and "onehalf m'egacycles at theedge 'ofthe intermediate frequency band separated from 3llfbyapproximately one-half megacycle. then with a circuit Q of '100f0rthe'resonant circuits and "52, the ratio of unwanted to wanted signal isabout .93 to 7 M13 per cent inthese circuits. This figure is arrived atby assuming atypical case and assigning arbitrary values to "the chartof'FigureG.

Assuming that the total frequency band in the intermediate frequencyamplifier 39extends from 8 megacycles to -14 megacycles and that thesound accompaniment utilizes a'necessary band'at'825 megacycle's, thevertical synchronizing signal may lie at Imegacyclesabove this value orat 13.5 megacycles: "The horizontalv synchronizing signal may beselected to lie at one-halfimegacycle below this or at'13.0megacycles.The ratio of "the response of the tuned circuit 52 at 13.0mega'cyclesitofitheresponse at 13.5 megacycles 'is' the voltage acrossthe inductance at 13.0 'megacycles divided Tby the 'voltage' across 'the'ln"-' ductance at 13.5 megacycles which is equal to V 1 we) The abovewritten formula is=to be found in Termans text, Radio Engineering.

In this formula, Q is the reactanceto resistance ratio of the circuit52- of Fig. '8 and 'isirassigned a numerical valueof 100 in the caseassumed-for the purpose-of-illustration; V is lthe ratio of -th'efrequency of the horizontal to the 'frequencyof theverticalsynchronizingpulses, and in-the assumed case has a numericalvalue or ;965. W-hen the numerical values giventaresubstituted in:the

formula written above the-numerical result-is 0:13. This-means that 13per cent of the total signal applied tothe wdetector- '53 is anundesired portion of the horizontal synchronizing pulse which is notremoved by the tuned circuit- 52'. This is likewise .true .afor thecircuit 48 and: the dectector 49. The unwantedv signals. appearing inthe outputs of the detectors 249 and 53 may be readily removedby biasingithesesdeteetors so that thirteen percent of theramplitude'ofrtheappliedsignals are not detected.

Equalizing "pulses, as explained. above;.*are-'used in prior. arttelevisionssystems :to eliminate the eflect of the transientresponsewhichfollowscess sation of a horizontal synchronizing :pulsepreceding a vertical synchronizing pulsenthi's tramsient response havinga .difierent' effect ion "the wave form of the vertical-impulse forodd:and even lines, In thesystem of the present invention, the transientresponse following cessation of alhorizon-tal pulse .isso small as toibe-negeligible for reasons now to'be'demonstrated.

The building. up of sinusoidal l currents 'in resonant circuits has.been treated by -.a, number :of writers. One such=treatment iorthesingle mesh R, L, C circuit is .found inGuillemin, fCommunicationNetworks, vol. 1, pp, 118-120. There. it is shown that when a sinusoidalvoltage is suddenly impressed on. a resonant circuit containingresistance R, inductance L, .and'capacityC, the envelope of thesinusoidal current flowingdn the circuit builds up in accordance -with.the

quantity and the envelopemfactorv for the building up period becomes Inthe present case'the building up of theresponse to a suddenly appliedsinusoidal synchronizingpulseis not pertinent but rather the rate ofsubsidence of the response when .the sinusoidal synchronizing signal 'isremoved. In this case. the envelope of the response decreases inaccordance with the quantity where t is now measured from the end of thesynchronizing impulse.

Substituting in this expression, the frequency at resonance, which is13.5 megacycles, and the time, which is the reciprocalof twice thehori-v zontal scanning impulse frequency of the time intervalcorresponding to one-half of a line, the result is 10- which is so smallcompared with unity as to have a negligible effect on the succeedingvertical impulse. If a biased detector is used, so that 13% amplitude ofthe horizontal impulses is not detected, a clean separation of thevertical pulse is accomplished without the necessity of equalizingpulses. The vertical synchronizing signal may then be of short timeduration as shown on Figure of the drawings. Because of its shortduration, it need not be of the serrated type to insure continuedhorizontal synchronism. It is seen, therefore, that perfectsynchronization can be obtained with the system according to the presentinvention without the use of equalizing pulses and serrated framesynchronizing pulses of complicated waveform.

Figure 12 of the drawings discloses a modified receiver for reproducingvisual representations of pictures or objects from signals transmittedby a transmitter modified as shown in Figure 11 of the drawings.Referring to Figure 11, it will be seen that the transmitter disclosedthereby is similar to the transmitter of Figure 1 with the addition ofan amplitude modulator 58 which amplitude modulates the carrier after ithas been frequency modulated in the frequency modulated oscillator 59 bythe output of the mixing amplifier which consists of the compositetelevision signal including picture or video signal components andsynchronizing signal components. The frequency modulation process hasalready been described in connection with Figure 1 of the drawings andneed not be repeated here. Figure 13 of the drawings discloses thegeneral nature of the wave radiated from the antenna system 62 withoutshowing the frequency variations to any particular scale. The portion 63of the wave carrying the intelligence or video signal is ofsubstantially constant amplitude but varies in frequency in accordancewith the amplitude variations of the video signal from the pick-updevice 64. Upon occurrence of blanking in the video amplifier 66, thecarrier resumes and maintains its unmodulated frequency until thevertical and horizontal synchronizing impulses are applied to the mixingamplifier 6|. The horizontal and vertical pulses are preferabl of theform shown in Figure 5 of the drawings whereby the frequency excursionof the carrier for the horizontal and vertical impulses is made todifier by some frequency value, for example 0.5 m. 0., so that thesignals may be received and translated into visual representations by areceiver embodying features described in connection with Figure 8 of thedrawings. Horizontal synchronizing impulses from the horizontalsynchronizing impulse generator 68, in addition to being supplied to themixing amplifier 6!, are also supplied to the amplitude modulator 58mentioned above. The modulator 58 serves ,to amplitude modulate thecarrier in accordance with the horizontal synchronization signals asindicated by reference character ll The percentage of modulation is notcritical and need be only sufficient to enable detection by an amplitudedemodulator or detector of the usual type. As its effect is to beeliminated by a limiter in the receiver the percentage modulation shouldnot be so great as to necessitate excessive limiting.

Figure 12 of the drawings discloses a modified form of receiverembodying features of the invention which enable it to receive signalsfrom the transmitter disclosed in Figure 11 of the drawings. Referringto this figure, I3 is an antenna system which feeds oscillator-mixerstage 16. The oscillator-mixer stage generates an intermediate frequencywhich is amplified in the intermediate frequency amplifier TI. Theamplitude variations of the composite intermediate frequency signalsincluding the position H are removed in the limiter stage 78 in themanner usual in phase or frequency modulated receiving systems. Thelimiter 18 may, like the limiter 4| of Figure 8, be of any conventionaltype. The frequency modulation discriminator or wave detector l9converts the frequency variations of the carrier to amplitude variationssuitable for operating the picture tube 80. A video signal amplifier 8|may be used if desirable or necessary to amplify the video signals. Thevertical and horizontal synchronization pulses which are present in theoutput of the wave detector 19 do not affect the picture tube forreasons pointed above in connection with Figure 8 of the drawings.

The output of the intermediate frequency amplifier 11 is fed to theamplitude demodulator or detector 82 of the usual type which provideshorizontal pulses from the envelope of the portion II of the carrier forcontrolling the horizontal scanning signal generator 82. The generator83 functions in the same manner as the generator 5| in Figure 8 of thedrawings.

' Pulses for controlling the vertical scanning signal generator arederived from the frequency modulated carrier by a separating system 84which is similar in all respects to the system 41 of Figure 8 andcomprises a circuit 85, which is tuned to the frequency of the portion63 of the carrier. The tuned circuit 85 causes the carrier frequencyproduced by the vertical synchronizing impulse to be applied to theamplitude detector 86. The output of the detector 86 is applied to thevertical scanning wave generator 88 which corresponds in function to theWave generator 54 of Figure 12 of the drawings. There is no interferencefrom the horizontal signal in the vertical impulse detector, as it hasbeen completely removed by the limiter 18.

Signals radiated by the transmitter of Figure 11 may be received by areceive-r embodying the features of the receiver of Figure 8 of thedrawings when the frequencies representing the horizontal and verticalpulses in the intermediate frequency amplifier differ in frequency. Thecirgrating circuit.

thistway; it is advantageous :to provide horizontal and vertical.synchronizing. pulses..ofzthgsame magnitude or. amplitude an'd to:depend :uporndifs ferentiating. between. vertical. and horizontalpulses-by difference in waveform. To :docthis successfully,-iti isadvantageous ;to employ :aawave formrof the samegeneral :typeasislnormally employed; with amplitude: irequencytelevision systems. Thiswave :form is shownin Figs. .15 and 16 and includes theequalizingrpulses m9 and Ill9a.

Figure 14 of .the xlrawings :discloses .:a rmodified form ofreceiver-for receiving va carrier .wave whose frequency is modulated in;accordance .with picture signals and. whose amplitude I is modulated in.accordance with synchronizing signals; In contrast \withthesystemsdescrib-ed above,..th'e synchronizing. signals: must include.equalizing pulses. .since discrimination. therebetween must nowbeefiected on a wave-form basis. The transmitter which radiates signalssuitable. for reception by the receiver of Figure-.314 may :be similarto the transmitter. ofvFigure 11 with the addition, however; of zazmeansto.=amplitude modulate the carrier. in accordanceiwith theverticalsynchronizing pulses.- Since it 'is-undesirable to frequency-modulatethe carrier .during the synchronizing pulses, the.connectionsbetween'the. horizontal synchronizing pulse-gen.- erator andthe ;mixing amplifier and. between the vertical synchronizing pulsegenerator-and the mixing amplifier, are preferably omitted.

Figures 15and 16 show the;present standard television signal. A briefdescription :of the standard :signal which modulates the amplitude of'the carrier the portions thereof. representing synchronizing and.equalizing 1.puls'es will be givenin the following paragraphs. Theblanking pulses It)! occur'between thevideo signals H32 which correspond.tothevideo signals 26 of Figure 32 of :the drawings. The horizontalsynchronizingimpulses H33 occur 'iiuringthe blanking periods. Figure :16shows the .composite-signal which is derived when scanning of :an: even.field'of view is completed. Atblankingcpulse I06 occurs equivalent ;in:lengthtto several scanning lines 102. The vertical synchronizing pulseI61 occurs during the long blanking .pulse and serves to return-the beamoraother scanning .instrumentality "to the top oiithe' frame orfiel-d ofview inpreparation for scanning :of. the next field. It willbenotedcfrom Figure 16 that the next'vertical scanning t.;pulse .Hl'la isdisplaced one-half line with.,respect to the precedinghorizontal pulseand, th.ere.f.ore,,- equalizing pulses 1 59 and 109a are substituted forthe-horizontal pulses in Figurespl and 16 j'respectiv-ely. The

equalizing impulses-occur. at double the frequency of the horizontal.synchronizing pulses and. alternate equalizing impulses coincides intime with the horizontaldmpulses. It :will be seen from'an inspection.of Figures 215 and lb thata horizontal synchronizing. impulse 101- anequalizing 1 impulse; always occurs at :the same=time intervalbefore-the beginningofla vertical synchronizingimpulse. The verticalsynchronizing impulses therefore are always :of the :same waveform-:inthe:integratingcircuit of the synchronization detector andseparator H0 shown on Figure '14 of thedra'wings. This piece-of. appratus 1 I0 is of: thesame typelused infknown'television systems andincludes a-demodulator or detector, a differentiation circuit andanrlinte- The-last :named .;cir.cuits. serve the purposeof. separatingthe; horizontal fromithe vertical synchronizing .impulses Dnfthe.:basissoi wave form. The impulses derived from the separating circuitsare :applied...to the horizontal sweep generator H2 and the verticalsweep generator I M respectively. These sweepgenerators [I2 and H4function in the usual manner-to supply currents or-voltages-to-thedeflecting 'coils or plates ofthe picture tube I I6 suitable forcontrolling the scanning -;;beam of the I picture tube H6.

The-signal from the intermediate frequency amplifier, which includesfrequency variations representing 'thevideo signal is, passed *to,thelimiter which. efiectively removes. the amplitude changes representingthe horizontal and vertical synchronizing impulses. The output:v of',the limiter which contains, only frequency variations is demodulatedin the discriminatorand is applied'to control the scanning beam of thepicture tube H6 or other scanning instrumentality for reproducing thepicture. g

While. the invention has beensdescribedgand explained in detail, in,connection with, several illustrative embodiments thereof,,.-it is to beunderstood that the invention maybe embodied in other formsand,itheref.ore,-the invention isnot limited ,exceptas indicatedlay-the. terms-and scope of the ,appendedclaima- From the foregoing, it.will, be .seen that .there has been provided.v a newt-system oftransmitting picture signals byway of a frequency-modulated signal.carrier. .wave and .it has been. showmthat such. transmission hasconsiderable advantages over the-conventional transmission of amplitudemodulated waves. Itv has alsobeenshOWnthat the. same signal carrier'wavezcan alsosbepfrequency-modulated by synchronizing signals, wherebyperfect synchronization can be-obtained bymeans-of signals which aremuch:simpler -than those required for. satisfactory synchronizationwhen. conventional "carrier amplitude modulation is used. Finally, therehaveibeenpprovided systems in. which the frequency -of "the signalcarrier wave is modulated in accordance with the :picture signals whileeither-its frequency-or amplitude, or both, are modulated in accordancewith the -1ine' or field synchronizing signals or both.

I claim:

1. Ida televisionzreceiver. adapted :to receive" a. signal jcarri-erWave havinggits .frequency modulated withina-predetermined range in.accord-.- ance with video signals and having its frequency shifted totwo different frequencies outsidesaid rangein accordance 'with'line andfield synchronizing signals, in combination; means for receivingesa'idsignal carrier wave, frequency discrimihating means for deriving videosignals from said characteristic of line synchronizing signals; a

second frequency discriminating; ircuit substantially responsive to'acarrier-frequency outside of said range characteristic of said fieldsynchronizing signals saidfirst and second circuits hav- 'ingappliedthereto video signal components and synchronizingsignal components,means for deriving'line synchronizing signals from said-first circuit,..means for deriving field "synchronizing signals from; said second.circuit; :and means for controlling .zsaidl :pic.t.ure:reproduci-ng lmeans 1.111,

accordance with said line and field synchronizing signals.

2. In a television receiver adapted to receive a signal carrier wavehaving its frequency modulated within a predetermined range inaccordance with video signals and having its frequency shifted to twodifferent frequencies outside said range in accordance with line andfield synchronizing signals, in combination, means for receiving saidcarrier wave, means for converting said wave into a secondfrequency-modulated carrier wave of intermediate frequency, frequencydiscriminating means for deriving video signals from said second carrierwave, picture reproducing means controlled by said video signals, afirst frequency discriminating circuit substantially responsive only tothe frequency of said second carrier wave characteristic of linesynchronizing signals, a second frequency discriminating circuitsubstantially responsive only to the frequency of said second carrierwave characteristic of field synchronizing signals, means for applyingsaid second carrier wave including video signal components andsynchronizing signal components to said first and said second circuits,separate demodulating means coupled to said first and said secondcircuits for separately deriving therefrom line and field synchronizingsignals, and means for controlling said picture reproducing means inaccordance with said synchronizing signals.

3. In a television receiver adapted to receive a, signal carrier wavefrequency modulated in accordance with video signals, having frequencycomponents within a predetermined range of frequencies, and having itsfrequencies shifted to two different frequencies outside said range inaccordance with line and field synchronizing signals, in combination,means for receiving said signal carrier wave, frequency discriminatingmeans for deriving video'signals from said frequency modulated carrierwave, picture reproducing means controlled by said video signals, afirst frequency discriminating circuit substantially responsive only toa carrier frequency characteristic of line synchronizing signals, asecond frequency discriminating circuit substantially responsive to acarrier frequency characteristic of said field synchronizing signals,means for applying video signal components and synchronizing signalcomponents to said first and second circuits, demodulating means forderiving line synchronizing signals from said first circuit,demodulating means for deriving field synchronizing signals from saidsecond circuit, and means for controlling said picture reproducing meansin accordance with said line and field synchronizing signals.

4. In a television receiver for the reception of a signal modulatedcarrier wave having its frequency modulated within a predetermined rangein accordance with picture signals and having its 18 frequency shiftedto two different frequencies outside said range in accordance with lineand field synchronizing signals, the combination of a tuner forreceiving such carrier wave, a limiter for limiting said receivedcarrier wave to a predetermined amplitude, a frequency discriminator forderiving picture signals from said limited carrier wave, a picturereproducer controlled by said derived picture signals, a first frequencydiscriminating circuit substantially responsive only to a carrierfrequency outside of said range characteristic of line synchronizingsignals, a second frequency discriminating circuit substantiallyresponsive only to a carrier frequency outside of said rangecharacteristic of said field synchronizing signal, connections betweensaid limiter and said first and second circuits for impressing thereonsaid limited carrier waves, and line and field sweep devices connectedrespectively with said first and second circuits and associated withsaid picture reproducer for controlling said picture reproducer inaccordance with said line and field synchronizing signal.

CHALON WESLEY CARNAHAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,212,968 Finch Aug. 27, 19402,086,833 Walton July 13, 1937 2,186,898 DHumy Jan. 9, 1940 1,941,068Armstrong Dec. 26, 1933 2,075,071 Usselmab Mar. 30, 1937 2,254,435Loughren Sept. 2, 1941 2,296,919 Goldstine Sept. 29, 1942 2,342,943 KellFeb. 29, 1944 1,548,895 Mertz Aug. 11, 1925 2,290,229 Finch July 21,1942 2,289,157 Whitaker Jul 7, 1942 2,326,740 Artzt Aug. 17, 19432,290,517 Wilson July 21, 1942 2,084,700 Ogloblinsky June 22, 19372,293,233 Wheeler Aug. 18, 1942 FOREIGN PATENTS Number Country Date433,295 Great Britain Aug. 6, 1935 OTHER REFERENCES "Radio Facsimile bySub-Carrier Frequency Modulation, by R, E. Mathes and J, N. Whitaker. R.C. A. Review, October, 1939.

The Service Range of Frequency Modulation, by M. G. Crosby, R. C. A.Review, January, 1940.

Electronics for February 1940, pages 26 and 30-32, article by C. W.Carnahan entitled Frequency Modulation. Magazine published byMcGraw-Hill Publishing Co., New York, N. Y.

R. C. A. Review, July 1940, vol. 5, No. 1 (pages 31 to 50).

